It is known that such an air data system is intended for measuring air data making it possible to determine values of parameters such as the altitude of the aircraft, its speed, etc. For this purpose, said air data system comprises pressure sensors that are connected by tubes to probes that pass through the fuselage of the aircraft.
Generally, a civil transport airplane comprises three air data systems of this type, of which:                a first provides data which are displayed in the cockpit, for the attention of the pilot;        a second provides data which are also displayed in the cockpit, but for the attention of the co-pilot; and        a third is used as backup to cope if necessary with the failure of one of said first two systems.        
For obvious safety reasons, these three air data systems must be made and must operate in a manner which is completely independent of one another, since otherwise the failure of one would have repercussions on the measurements of the other systems.
Moreover, it is known that human errors, in particular during airplane maintenance operations, may give rise to the failure of one or more of the pressure sensors of such an air data system.
For example, the personnel in charge of performing an operation of washing the airplane frequently stick a piece of adhesive tape onto each of the probes of the air data system, so as to prevent water from penetrating into the tube during said washing. If they forget to remove one of said pieces of adhesive tape after the washing, the corresponding pressure sensor will fail during the next flight of the airplane, since it will be unable to measure the pressure of the air outside the fuselage. It will in fact measure the pressure of the air in the tube which is closed, at its end on the probe side, by the adhesive piece.
Another exemplary failure relates to the case where maintenance personnel disconnect the tube and the pressure sensor at the level of an appropriate connector, so as for example to clean the inside of this tube. If they forget to reconnect the tube to the pressure sensor after carrying out the maintenance operation, said pressure sensor will also fail, since it will measure the pressure of the air inside the fuselage instead of measuring the pressure of the air outside the fuselage.